the effect of flexible chains on the orientation

Japan Advanced Institute of Science and Technology

JAIST Repositoryhttps://dspace.jaist.ac.jp/

Title

The effect of flexible chains on the orientation

dynamics of small molecules dispersed in polymer

films during stretching

Author(s)

Nobukawa, Shogo; Aoki, Yoshihiko; Fukui,

Yoshiharu; Kiyama, Ayumi; Yoshimura, Hiroshi;

Tachikawa, Yutaka; Yamaguchi, Masayuki

Citation Polymer Journal, 47(4): 294-301

Issue Date 2014-12-24

Type Journal Article

Text version author

URL http://hdl.handle.net/10119/12868

Rights

Copyright (C) 2014 The Society of Polymer

Science, Japan. Shogo Nobukawa, Yoshihiko Aoki,

Yoshiharu Fukui, Ayumi Kiyama, Hiroshi Yoshimura,

Yutaka Tachikawa and Masayuki Yamaguchi, Polymer

Journal, 47(4), 2014, 294-301.

http://dx.doi.org/10.1038/pj.2014.126

Description

The effect of flexible chains on the orientation dynamics of small

molecules dispersed in polymer films during stretching

Shogo Nobukawa†*, Yoshihiko Aoki†, Yoshiharu Fukui†, Ayumi Kiyama†, Hiroshi

Yoshimura‡, Yutaka Tachikawa§, and Masayuki Yamaguchi†

†Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa

923-1292, Japan,

‡DIC Corporation, Chiba Plant, 12, Yawata-kaigandori, Ichihara, Chiba 290-8585,

Japan,

§DIC Corporation, Central Research Laboratories, 631, Sakado, Sakura, Chiba

285-8668, Japan

Running head: Orientation of small molecule in polymer film

*Corresponding authors:

Shogo Nobukawa

E-mail: [email protected]

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1

Asahidai, Nomi, Ishikawa 923-1292, Japan, Tel: +81-761-51-1626

Orientation of small molecule in polymer film 1

ABSTRACT

The effect of flexible chain on orientation dynamics of a small additive

molecule in cellulose acetate propionate (CAP) films is investigated during stretching

by means of measurements of birefringence and polarized Fourier-transform infrared

(FT-IR) spectrum. The orientation birefringence is enhanced by the additives such as

ethylene 2,6-naphthalate (C2Np) and hexamethylene 2,6-naphthalate (C6Np) oligomers,

suggesting that the additives orient parallel to the CAP chain due to an intermolecular

orientation correlation called as the nematic interaction (NI). At a high drawing

temperature, the orientation of C2Np is stronger than that of C6Np, indicating that the

flexible alkyl chain reduces the intermolecular NI with CAP. In contrast, C6Np orients

to the stretching direction rather than C2Np at a low temperature. The polarized FT-IR

spectra show that the orientation of rigid part in C6Np is delayed from that of the

flexible group and matrix CAP chain. Consequently, the contribution of C6Np to

birefringence and its wavelength dependence become stronger with increasing the draw

ratio.

Keywords: Additive orientation / Birefringence / Flexible chain / Nematic interaction /

Stretching

Nobukawa et al. 2

INTRODUCTION

Orientation dynamics of polymer chains are strongly related to various kinds of

properties such as mechanical, optical, thermal, and dielectric properties.1-5 In

particular, the chain orientation generates an optical anisotropy, namely, birefringence,

which is important for optical applications, such as liquid crystal displays, electro

luminescence displays, and optical pick-up lenses. In order to improve the

performance of these devices, both birefringence value and its wavelength dependence

are necessarily controlled. For example, quarter-wave plate, which is one of the most

important optical films, is required to exhibit a retardation of a quarter of wavelength

for the wide range of visible lights. However, in general, such optical properties

cannot be provided in a conventional film using a single polymer component.

When polymer films are stretched beyond a glass transition temperature (Tg),

the orientation birefringence, ∆n, is induced. ∆n is defined to be the difference

between the two refractive indices in the directions parallel and perpendicular to the

stretching direction, i.e., //n and ⊥n , respectively, ⊥−=∆ nnn // . The orientation

birefringence ∆n is proportional to the degree of the chain orientation, F, as follows.6

( ) ( )Fnn λλ 0∆=∆ (1)


Here, ∆n0 is an intrinsic birefringence where the anisotropic molecule perfectly orients

to the stretching direction. As represented in equation 1, ∆n0 is dependent on the

wavelength, λ, while F is independent. Therefore, for polymer films composed of

single component, the wavelength dependence of ∆n, ( ) ( )0λλ nn ∆∆ , cannot be

changed by the chain orientation as represented by,

( )( )

( )( ) .

00

0

0

constnn

nn

=∆∆

=∆∆

λλ

λλ (2)

In order to control the birefringence and its wavelength dependence for optical

films, a combination of double or multi components having different wavelength

dependence is applicable. Various techniques such as blending with other polymers7, 8

or small molecules9-12, copolymerization13-15, and sheet piling16 have been considered.

For the industrial applications, blending with small molecules is easier and more

suitable than the other methods due to the following reasons. For polymer blending

and copolymerization, the number of combination of materials is limited due to the poor

miscibility of polymer pairs. For lamination, the thermal expansion mismatch between

polymer sheets restricts the temperature range of use.

The orientation birefringence of the blend, ∆nblend, is expressed by a simple

addition rule given by,

Nobukawa et al. 4

( ) ( )( ) iii

iblend

Fn

nn

∑∑

∆=

∆=∆

λφ

λλ0

(3)

where φi, 0in∆ , and Fi are volume fraction, intrinsic birefringence, and orientation

function for component i. As shown in Figure 1, the birefringence and its wavelength

dependence of the blend are determined by the component birefringences. Especially,

concentration and orientation function of the components contribute to the birefringence

as represented by equation 3. Since the small molecules reduce the thermal resistance

of polymer films due to the plasticization effect, the additive content is limited.

Contrary, the additive orientation is originated by an intermolecular orientation

correlation, which is called as a nematic interaction (NI).17, 18 The orientation function

of additives, Fadd, is proportional to that of matrix polymer, Fpoly, suggesting that the

wavelength dependence cannot be controlled by the stretching condition if the

orientation dynamics of additives are completely coupled with that of the matrix

polymer chain. However, if the additive orientation is independently generated from

the matrix polymer, the wavelength dependence can be modified for the blend films

with the same composition.


[Figure 1]

Choudhury et al.19 investigated the orientation dynamics of poly(ethylene

terephthalate) (PET) with high molecular weight by using 1H-13C cross-polarization

magic-angle-spinning (CP/MAS) NMR spectroscopy. According to them, ethylene

part in PET exhibits 10 times shorter relaxation time than phenyl ring, indicating that

the orientation of the flexible chain is induced by stretching prior to the orientation of

the rigid part. Therefore, for small molecules with similar structure to PET, the

flexible and rigid parts might exhibit different orientation time in polymer films during

stretching, leading to the control of the wavelength dependence for birefringence.

In this study, the effect of alkyl chains on the orientation dynamics of small

additives in polymer films during stretching is investigated. Two types of alkyl

naphthalate as model compounds are mixed with cellulose acetate propionate (CAP),

which is used as a model polymer for optical films.20 Especially, the detail of

additive orientations in CAP films is investigated by using the birefringence

measurement and Fourier-transform infrared (FT-IR) spectroscopy.

EXPERIMENTAL

Samples

Nobukawa et al. 6

CAP (Figure 2) was produced by Eastman Chemical Company (TN, USA).

The weight-average and number-average molecular weights (Mw and Mn) of CAP were

2.1 × 105 and 7.7 × 104, respectively, determined by a gel-permeation-chromatography

(GPC, HLC-8020 Tosoh, Japan) with a polystyrene standard. Degrees of substitution

for acetyl and propionyl groups per a pyranose unit of CAP are 0.19 and 2.58,

respectively. Ethylene 2,6-naphthalate) and hexamethylene 2,6-naphthalate oligomers

(C2Np and C6Np, Figure 2) used as additives were synthesized from alkane diol and

dimethyl naphthalene 2,6-dicarboxylate (DMNDC) as following reactions. DMNDC

and ethylene glycol (EG) (or 1,6-hexamethylene diol, HD) with an excess molar

quantity against DMNDC were mixed in a flask. After adding tetraisopropyl titanate

as a catalyst for the esterification reaction into the flask, the mixture was stirred at

200 oC for half a day under a nitrogen atmosphere. Finally, by removing the residual

EG (or HD) at 200 oC in vacuo, C2Np or C6Np was obtained. The number-average

molecular weight (Mn) was calculated from the fraction of hydroxyl group at the chain

end in a molecule, which was estimated using a method of end-group determination

with potassium hydrate. The melting temperature (Tm) of the additives was evaluated

by a differential scanning calorimeter (DSC-822e, Mettler Tolede Int. Inc., Switzerland).

The values of Mn and Tm are summarized in Table 1. As shown in the table, the

degrees of polymerization of C2Np or C6Np are slightly different (1.3 and 1.6,


respectively).

Blends of CAP and C2Np (or C6Np) at a weight ratio of 100/10 wt/wt were

prepared by using a 60 cc batch-type internal mixer (Labo-plastmil, Toyoseiki, Japan) at

200 oC for 6 min with the blade rotation speed of 30 rpm. The preparation condition

was determined as explained in our previous paper.21 In order to avoid chemical

reactions such as hydrolysis degradation and trans-esterification, CAP was dried in

vacuo at 80 oC for 2 hours before melt-mixing. After being kept in a vacuum oven at

room temperature for at least one day, the blend samples were compressed into sheets

with a thickness of 20-30 µm at 200 oC for 5 min under 10 MPa by a

compression-molding machine (Table-type-test press SA-303-I-S, Tester Sangyo, Japan)

and were subsequently cooled down at 25 oC for 5 min.

[Figure 2]

[Table 1]

Measurements

Dynamic mechanical analysis (DMA) for the sample films was carried out to

determine tensile storage and loss moduli (E' and E'', respectively) at 10 Hz as a

function of temperature by a tensile oscillatory rheometer (DVE-E4000, UBM, Japan)

Nobukawa et al. 8

from 25 to 180 oC with a heating rate of 2 oC min-1.

DSC was operated to evaluate the crystallinity in CAP and CAP/additive films

during a first heating process with a heating rate of 10 oC min-1.

A hot-stretching test for the films was carried out with a strain rate of 0.05 s-1

by using a tensile drawing machine (DVE-3, UBM, Japan). The stretching

temperatures were determined from the DMA data, where the tensile modulus is 10 or

100 MPa at 10 Hz. The films were immediately quenched by cold air blowing after

stretching to avoid relaxation of molecular orientation.

The stretched films were kept in a humidic chamber (IG420, Yamato, Japan) at

25 oC and 50 %RH for one day in order to prevent moisture effect on birefringence data

as reported in a previous paper.22 During the stock process in the chamber, the chain

orientation of CAP does not relax because glass transition temperatures (Tg’s) are much

higher than the storage temperature even for CAP/additive blends. The birefringence

of the drawn films was measured as a function of wavelength by using an optical

birefringence analyzer (KOBRA-WPR, Oji Scientific Instruments, Japan).23

In order to evaluate orientation functions (F) for CAP, C2Np and C6Np in the

stretched films, polarized Fourier-transformed infrared (FT-IR) spectra were measured

over the scan range of 400 - 4000 cm-1 with a resolution of 4 cm-1 at room temperature

by using an FT-IR spectrometer (Jasco. Inc., Japan) equipped with a wire-grid polarizer.


The detail of the measurement for molecular orientation is explained in supplementary

information.

RESULTS and DISCUSSION

Miscibility and plasticization of CAP by additives

Figure 3 shows the dynamic mechanical property of CAP and CAP/additive

blends in temperature range of 0 to 180 oC. The glass transition temperature (Tg)

defined as the peak temperature of loss modulus E '' was determined to be 146, 119 and

116 oC for bulk CAP, CAP/C2Np and CAP/C6Np, respectively, as listed in Table 2.

Both blends showed broader glass transition than bulk CAP, meaning that the relaxation

time distribution for the glass transition becomes wider by the addition of C2Np and

C6Np. This phenomenon, which is explained by a concentration fluctuation theory,

has been reported in many polymer blends including plasticized polymers.24-26 The

distribution of concentration in the systems is related to the broadness of the glass

transition. Furthermore, the reduction of Tg by C2Np and C6Np on CAP are the same

degree, which might be due to the similarity of chemical structure and molecular weight

as shown in Figure 2 and Table 1.

[Figure 3]

Nobukawa et al. 10

[Table 2]

In the rubbery region above Tg, CAP/additive blends show the same level (~

10 MPa) of storage modulus E ' with the bulk CAP. In general, the rubbery plateau

modulus for crystalline polymers such as cellulose acetate is 100 MPa.27, 28 However,

as shown in Figure 4, DSC curves for the films show no melting peak, which could

appear from 150 to 200 oC for CAP used in this study23, suggesting that the crystalline

content is negligibly small for CAP in bulk and blend films.

[Figure 4]

Stress-strain behavior of CAP and CAP/additive films

Stretching polymer films induces chain orientation, which originates tensile

stress σ and birefringence ∆n. According to the stress-optical rule represented in

equation 4, the following relation between ∆n and σ is obtained.

σCn =∆ (4)

Considering equation 1, σ is proportional to the orientation function F as well as ∆n, i.e.,


the chain orientation can be controlled by the stress level.

Prior to stretching CAP and CAP/additive films, two drawing temperatures

(Tdraw) were determined from storage modulus in Figure 3 following the previous

paper.20 As shown in Table 2, the gap between Tdraw and Tg becomes larger by

additives because the glass transition is broadened as seen in Figures 3 and 4. Figure 5

shows the stress-strain (S-S) curves of CAP and CAP/additive films at two Tdraw’s

(T10MPa or T100MPa). For CAP films, the stress levels are much different between T10MPa

and T100MPa. However, as explained later, the orientation function of CAP is similar

between the two draw temperatures while it depends on the draw ratio, meaning that the

orientation behavior of CAP chain is insensitive to temperature within the experimental

condition in this study. This might be because the tensile stress at the lower

temperature (T100MPa) contains the glassy stress related to the local distortion in CAP

chain as well as the rubbery stress originated from the chain orientation, as reported by

Maeda and Inoue.29 Consequently, the rubbery stress is almost similar between the

two draw temperatures, meaning that the difference in orientation function of CAP is

small.

[Figure 5]

Nobukawa et al. 12

By comparing S-S curves in Figure 5, the three films exhibit the same level of

tensile stress at T10MPa. Contrary, at T100MPa, the stress value for CAP/C2Np is slightly

larger than those for CAP and CAP/C6Np. However, the orientation functions of CAP

in bulk and blend films is almost similar because the rubbery stresses are not so

different, as mentioned above for pure CAP. The same level of orientation function

suggests that the orientation birefringence of CAP is similar between bulk and blend.

Therefore, the difference in orientation birefringence between CAP and CAP/additive is

originated from the additive orientation as previously discussed.20

Orientation birefringence of stretched CAP/additive films

Figure 6 shows the orientation birefringence ∆n of CAP and CAP/additive

films stretched at two drawing temperatures. In the figure, ∆n values of all samples

monotonically increase with the draw ratio as well as the tensile stress σ shown in

Figure 5. By comparing Figures 5 and 6, however, the curves of ∆n against draw ratio

are obviously different from that of σ for bulk and blend. In general, since both σ and

∆n of amorphous polymers at the rubbery state are originated from the chain orientation,

the two curves against draw ratio should be the same as predicted from Equation 4. In

contrast, according to Yamaguchi et al.23, ∆n for CAP is generated by the alignment of

substitution groups, which are acetyl and propionyl groups, because the polarizability


anisotropy of a pyranose ring in the main chain is negligibly small. Furthermore, since

the alignments of acetyl and propionyl groups are not completely cooperative with the

main chain orientation at higher draw ratio, the curves of σ and ∆n were different for

CAP and CAP/additive blends, as shown in Figures 5 and 6.

[Figure 6]

In Figure 6, the ∆n values of CAP/additive films are larger than that of the CAP

film, indicating that the additive molecules contribute ∆n as represented by equation 3.

It is shown in Figure 6(A), the effect of C2Np is stronger than that of C6Np for ∆n at

T10MPa. However, Figure 6(B) at T100MPa indicates that the enhancement by C6Np is

almost similar to that by C2Np. This result demonstrates that the contribution of the

additives to ∆n depends on the drawing temperature. As reported in our previous

papers20, 21, the enhancement of birefringence is generated by the additive orientation,

which is induced by the chain orientation of surrounding polymers, i.e., the

intermolecular NI. Therefore, the effect of the drawing temperature on the orientations

of C2Np and C6Np in CAP is compared.

Figure 7 compares the wavelength dependence of ∆n for CAP and

CAP/additive films stretched at various draw ratios. As reported by Yamaguchi et al.23,

Nobukawa et al. 14

the CAP film shows extraordinary wavelength dispersion, in which ∆n increases with

wavelength, and the slope of curve becomes larger with increasing draw ratio. On the

other hand, CAP/additive films except for CAP/C6Np at T10MPa exhibit ordinary

wavelength dispersion, where ∆n decreases with wavelength. Furthermore, the

wavelength dependence is affected by the additive species. In the case of CAP/C2Np

film, the wavelength dependence of ∆n is independent of the draw ratios in this study.

Contrary, for CAP/C6Np film, the wavelength dependence slightly changes from

extraordinary to ordinary with increasing draw ratio, suggesting that the additive

orientation is slower than the chain orientation of CAP during stretching.

[Figure 7]

According to equations 2 and 3, the wavelength dependence of ∆n in binary

blend films is determined from the ratio of two components. Since the wavelength

dependence for CAP becomes stronger with the draw ratio, the result for CAP/C2Np in

Figure 7(B), where the wavelength dependence does not change, suggests that the

contribution of C2Np to birefringence is enhanced more at larger draw ratios.

Additionally, the wavelength dispersion for CAP/C6Np was changed from extraordinary

to ordinary at large draw ratios, meaning that the contribution of C6Np becomes


stronger during stretching as in the case of CAP/C2Np. These results might be related

to the difference in the time/strain required for the orientation of CAP chains and

additives because it depends on the temperature. In order to discuss the orientation

dynamics of C2Np and C6Np in CAP films at two drawing temperatures, molecular

orientation is discussed by polarized FT-IR spectra in the following sections.

Molecular orientation of CAP and additives in stretched film

Polarized FT-IR spectroscopy can investigate orientation function, F, of CAP

and additives in stretched films by using the following equation.

21

21

0

0

+−

−+

=RR

RRF (5)

Here, R is a ratio of peak intensities, A || / A⊥. The subscripts of peak absorbance, A,

represent parallel and perpendicular axes to the stretching direction. R0 = 2cot2β,

where β is the angle between the transition moment corresponding to the absorption

band and the chain axis. The orientation function of CAP, FCAP, was evaluated by the

peaks at 806 and 884 cm−1. Orientations of aromatic part in C2Np and C6Np were

investigated from the peak at 772 cm−1. The orientation function of the hexyl chain in

C6Np was also estimated from the peak at 2860 cm−1. The ethylene chain orientation

Nobukawa et al. 16

of C2Np cannot be evaluated because the peak of C-H stretching mode appears at the

same wavenumber with that for CAP. The analysis of F values is described in

supplementary information.

Figure 8(A) shows the orientation function of CAP, F884, which was calculated

from the peak at 884 cm−1. The degree of chain orientation, F884, monotonically

increases with the draw ratio, corresponding to the stress-strain curve in Figure 5,

because the tensile stress is originated from the chain orientation as mentioned before.

Fig.8(A) represents the same level of chain orientation for CAP in the bulk and blend

although the degrees of orientation are slightly different between T10MPa and T100MPa.

[Figure 8]

Figure 8(B) shows the relation between draw ratio and orientation function at

772 cm−1, F772, which represents the orientation of aromatic group in C2Np and C6Np.

As demonstrated in the figure, the orientation of C2Np is stronger than that of C6Np at

T10MPa while it is slightly weaker at T100MPa. The difference in F772 between C2Np and

C6Np at two temperatures corresponds to the result for birefringence in Figure 6. As

mentioned before, the additive orientation (Fadd) is induced by the chain orientation of

surrounding polymer (Fpoly) via NI18, as represented by the following equation.


polypolypolypoly

addpolypolyadd FF

−=

,

,

1 εφεφ

(6)

Here, φ is a volume fraction and εi,j is the NI parameter between components i and j.

Since φpoly and εpoly,poly are constants for the samples in this study, the degree of additive

orientation Fadd (= F722) is determined by only the intermolecular NI represented by

εpoly,add. Therefore, the additive orientation due to the NI can be discussed with the

ratio, Fadd/Fpoly (=F772/F884).

Orientation dynamics of C2Np and C6Np induced by NI

Prior to investigate the orientation dynamics of the additives in CAP, the effect

of molecular weight is commented. In our previous paper20, the effect of molecular

weight on the orientation of alkyl terephthalate oligomers in CAP was discussed.

According to the paper, for higher degree of polymerization (n) than 2, the orientation

function of the additive oligomers becomes larger with increasing the n value.

However, for lower value of n (1 or 2), the additive orientation is almost similar.

According to the result, in this study, since the n values of C2Np and C6Np are lower

than 2 (1.3 and 1.6, respectively), the effect of n on the orientation dynamics can be

Nobukawa et al. 18

ignored.

In order to discuss the orientation dynamics of C2Np and C6Np in CAP during

stretching, F772 / F884 was calculated at various draw ratios as shown in Figure 9. The

ratio for CAP/C2Np exhibits the same dependence on the draw ratio at T10MPa and

T100MPa, and slightly increases with draw ratio. This result suggests that the C2Np

orientation is simply generated by the intermolecular NI with the chain orientation of

CAP. On the other hand, F772 / F884 for CAP/C6Np is obviously dependent on the

drawing temperature. The ratio, F772 / F884 at T100MPa is larger than that at T10MPa at

high draw ratios, leading the difference in the effects of C6Np at the two draw

temperatures shown in Figure 6. In addition, the ratio at T100MPa increases with the

draw ratio, indicating that the orientation of the aromatic group in C6Np, which

corresponds to the FT-IR peak at 772 cm-1, is delayed from the chain orientation of CAP

at T100MPa.

[Figure 9]

Choudhury et al.19 investigated chain dynamics of poly(ethylene terephthalate)

(PET) with high molecular weight by using 1H-13C cross-polarization

magic-angle-spinning (CP/MAS) NMR spectroscopy. They explained that the


relaxation time of the phenyl ring in PET is 10 times as long as that of the ethylene

group, suggesting that the rigid and flexible parts have the different time/strain required

for the orientation. Since the additive molecules in this study also have similar

structure to PET, the above result is applicable for the discussion of orientation

dynamics in this study.

The orientation of flexible part in C2Np cannot be investigated because its

FT-IR peak of C-H stretching band is overlapped with that in CAP. On the other hand,

the orientation of flexible hexa-methylene group in C6Np can be evaluated since the

peak locates at different wavenumber from that in the matrix polymer. The orientation

functions of rigid and flexible parts in C6Np, which are estimated from FT-IR peaks at

772 and 2860 cm−1, respectively, are shown in Figure 10. At higher drawing

temperature (T10MPa), F772 and F2860 are comparable, meaning that the flexible and rigid

parts in C6Np have the same orientation time in stretched CAP film. However, at

lower temperature (T100MPa), F772 becomes close to F2860 with increasing the draw ratio,

implying that the aromatic group in the C6Np molecule has longer relaxation time than

the alkyl chain. This orientation behavior of C6Np is similar to the chain dynamics of

PET reported by Choudhury et al.

[Figure 10]

Nobukawa et al. 20

As shown in Figure 9, the orientation of C6Np at low temperature (T100MPa)

was stronger than that at high temperature (T10MPa). This phenomenon was also

reported for other types of additives in our previous paper.20 The orientation behavior

of additives is explained as follows. According to the nematic interaction (NI) theory,

the additive orientation is induced by the chain orientation of matrix polymers due to

the intermolecular NI, i.e., orientation coupling. Therefore, the additive orientation

(Fadd) should be cooperative with or delayed from the chain orientation of matrix

polymer (Fpoly), meaning that the ratio of orientation functions, Fadd/Fpoly becomes

smaller at lower temperatures because the relaxation time of additives becomes longer.

However, at low temperature, the glassy stress generates the local chain distortion of

polymers, which might enhance the flexible chain alignment such as the trans formation

of polyethylene chain. Actually, in the experimental data, the orientation of the

flexible part (hexa-methylene group) in C6Np at T100MPa is stronger than that at T10MPa.

For C2Np, the flexible part is too short to induce the orientation at T100MPa. The

alignment of flexible part enhances the orientation of rigid part in C6Np, especially at

T100MPa as shown in Figure 9.

Based on the above discussion, a following model is considered to explain the

orientation dynamics of C6Np in CAP film. Firstly, the chain orientation of CAP


induces the alignment of flexible parts in C6Np. At low temperature near Tg, the local

chain distortion originated from the glassy stress also generates the flexible chain

orientation. Secondly, the rigid (aromatic) part orients after the flexible part

orientation due to the difference in orientation relaxation times. As the result, the

wavelength dependence of birefringence for CAP/C6Np was changed during stretching

as shown in Figure 7(C). In general, the wavelength dependence of orientation

birefringence of optical polymers is hardly controlled by processing conditions.

However, as discussed in this study, the wavelength dependence in polymer blend can

be modified by adjusting the drawing conditions for the orientation dynamics of

additive molecules.

Conclusions

In order to control the wavelength dependence of orientation birefringence for

cellulose acetate propionate (CAP) film, we investigated the orientations of small

additive molecules in CAP films during stretching. In order to examine the effect of

flexible chains on the orientation dynamics, two types of additives, 2,6-ethylene

naphthalate (C2Np) and 2,6-hexamethyl naphthalate (C6Np) oligomers, were used in

this study.

The birefringence of CAP/additive film was larger than that of pure CAP film,

Nobukawa et al. 22

indicating that the additive orientation is generated due to an orientation correlation

represented by an intermolecular nematic interaction (NI). Furthermore, C2Np

improved the birefringence of CAP more than C6Np at higher drawing temperature.

However, at lower temperatures, C6Np showed stronger enhancement of birefringence

than C2Np. This result suggests that the orientation of C6Np is generated more than

that of C2Np at the lower temperatures. The wavelength dependence of birefringence

for CAP/C6Np film was gradually changed from extraordinary to ordinary, indicating

that the orientation dynamics of C6Np is not cooperative with the chain orientation of

CAP although the orientation is induced by the NI.

From polarized FT-IR spectra, the orientation function of CAP, C2Np and

C6Np were evaluated. Especially, for C6Np, orientations of the hexa-methylene group

and the aromatic ring were not coupled during stretching at lower temperatures. This

result suggests that the flexible part in C6Np oriented prior to the alignment of the rigid

part, i.e., the different orientation time of the flexible and rigid parts in alkyl naphthalate

molecules. By applying this phenomenon, birefringence and its wavelength

dependence of polymer films containing additives could be controlled even in the same

blend films.


Acknowledgement

This work was partly supported by a Grant-in-Aid for Young Scientists B

(25870268), and by a grant from the Kyoto Technoscience Center in 2013.

Supplementary information is available at Polymer Journal’s website.

Nobukawa et al. 24

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7. Saito, H. & Inoue, T., Chain orientation and instrinsic anisotropy in birefringence-free polymer blends. J. Poly. Sci. Part-B Poly. Phys. 25, 1629-1636 (1987).

8. Uchiyama, A. & Yatabe, T., Analysis of extraordinary birefringence dispersion of uniaxially oriented poly(2,6-dimethyl 1,4-phenylene oxide)/atactic polystyrene blend films. Japan J. App. Phys. Part-1 42, 3503-3507 (2003).

9. Tagaya, A., Iwata, S., Kawanami, E., Tsukahara, H. & Koike, Y., Zero-birefringence polymer by the anisotropic molecule dope method. App. Opt. 40, 3677-3683 (2001).

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11. Abd Manaf, M. E., Tsuji, M., Shiroyama, Y. & Yamaguchi, M., Wavelength Dispersion of Orientation Birefringence for Cellulose Esters Containing Tricresyl Phosphate. Macromolecules 44, 3942-3949 (2011).

12. Abd Manaf, M. E., Miyagawa, A., Nobukawa, S., Aoki, Y. & Yamaguchi, M., Incorporation of low-mass compound to alter the orientation birefringence in cellulose acetate propionate. Opt. Mater. 35, 1443-1448 (2013).

13. Iwasaki, S., Satoh, Z., Shafiee, H., Tagaya, A. & Koike, Y., Design and synthesis of zero-zero-birefringence polymers in a quaternary copolymerization system. Polymer 53, 3287-3296 (2012).


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Figure and Table legends Figure 1. Schematic representation for control of birefringence and its wavelength dependence by additives mixed with matrix polymer. Figure 2. Chemical structures of CAP and additives. Figure 3. Temperature dependence of tensile storage and loss moduli (E' and E'') for bulk CAP, and CAP/additive (100/10 wt/wt) films. The oscillatory frequency is 10 Hz and heating rate is 2 oC min-1. Figure 4. DSC curves of CAP and CAP/additive films during heating process with a heating rate of 10 oC/min. The arrows indicate the glass transition temperatures. Figure 5. Stress-strain curves of CAP and CAP/additive films at two draw temperatures where E' is 10 or 100 MPa. The strain rate is 0.05 s-1. Figure 6. Orientation birefringence ∆n of CAP and CAP/additive (100/10 wt/wt) films stretched with various draw ratios at T10MPa and T100MPa. Figure 7. Comparison of wavelength dependence for ∆n of CAP and CAP/additive films stretched with various draw ratios at two drawing temperatures. Figure 8. Orientation functions for (A) CAP and (B) additives in stretched films with various draw ratios at two drawing temperatures. Figure 9. Ratio of orientation functions for additive and CAP in blend films stretched with various draw ratios at two drawing temperatures. Figure 10. Ratio of orientation functions for rigid and flexible parts of C6Np in blend film stretched with various draw ratios. Table 1. Molecular weight (Mn), degree of polymerization (n), melting point (Tm), and intrinsic birefringence (∆n0) of additives. Table 2. Glass transition temperature (Tg) and drawing temperature (Tdraw) for CAP and CAP/additive blends.

Nobukawa et al. 28

Table 1. Molecular weight (Mn), degree of polymerization (n), melting point (Tm), and intrinsic birefringence (∆n0) of additives.

additive Mn a) n Tm / oC b) ∆n0 c)

C2Np 390 1.3 85 0.14 C6Np 643 1.6 97 0.081

a) estimated by a method of end group determination using potassium hydrate b) determined by DSC c) calculated by a molecular dynamics simulation with bond-polarizability parameter21


Table 2. Glass transition temperature (Tg) and drawing temperature (Tdraw) for CAP and CAP/additive blends.

Tg / oC a) Tdraw / oC b) (Tdraw − Tg)

E' = 10 MPa E' = 100 MPa CAP 146 163 (+17) 152 (+6)

CAP/C2Np 119 146 (+27) 127 (+8) CAP/C6Np 116 149 (+33) 133 (+17)

a) defined as a peak temperature of E'' in Figure 3 b) determined as a temperature where E' is 10 or 100 MPa

Figure 1 Nobukawa et al.

Bire

fring

ence

, ∆n

Component B Ordinary λ dependence

Component A Extraordinary λ dependence

Blend A + B

∆n

(λ) /

∆n

(λre

f) Wavelength, λ λ λref

1

B

A

A + B

Figure 2

O

OOO

CH2OR

ORRO

ROOR

CH2OR n

R = COCH3 (acetyl) = COCH2CH3 (propionyl)

(A)

OR

OH

O

OR

O

n

HO

R =

(CH2)2 (C2Np)

(CH2)6 (C6Np)

(B)

Nobukawa et al.

Figure 3

6

7

8

9

10

0 60 120 180

CAPCAP/C

2Np

CAP/C6Nplo

g [E

' /P

a], l

og [E

'' /Pa

]

Temperature / oC

TgE '

E ''

Tg

Nobukawa et al.

Figure 4

50 100 150 200

Temperature / oC

Exth

othe

rmic

CAP/C2Np

CAP

CAP/C2Np

10 oC/min

Nobukawa et al.


0

1

2

3

1.0 1.5 2.0 2.5 3.0

CAP

CAP/C2Np

CAP/C6Np

Stre

ss /M

Pa

Draw ratio

T10 MPa

(A)

0

1

2

3

4

5

6

1.0 1.2 1.4 1.6 1.8 2.0

CAP

CAP/C2Np

CAP/C6Np

Stre

ss /M

Pa

Draw ratio

(B)T

100 MPa


0

10

20

30

40

50

1.0 1.5 2.0 2.5 3.0

CAP

CAP/C6Np

CAP/C2Np

∆n

/ 10 -

4

Draw ratio

T10 MPa

(A)

0

10

20

30

40

50

1.0 1.5 2.0 2.5 3.0

CAP

CAP/C6Np

CAP/C2Np

∆n

/ 10 -

4Draw ratio

(B)

T100 MPa

Figure 7

0.6

0.8

1.0

1.2

1.52.02.53.0

T10 MPa

DR

CAP

Nobukawa et al.

0.6

0.8

1.0

1.2

400 500 600 700 800

1.52.02.5

∆n

/ ∆n

(589

nm

)

Wavelength /nm

T 100 MPa

DR

(A)

0.9

1.0

1.1

400 500 600 700 800

1.52.02.5

∆n

/ ∆n

(589

nm

)

Wavelength /nm

DR

T100 MPa

0.9

1.0

1.1

400 500 600 700 800

1.52.02.5

∆n /

∆n (5

89 n

m)

Wavelength /nm

DR

T100 MPa

(B) (C)

0.9

1.0

1.1

1.52.02.53.0

DR

CAP/C2NpT

10 MPa

0.9

1.0

1.1

1.52.02.53.0

T10 MPa

DR

CAP/C6Np


0

0.1

0.2

0.3

0.4

1.0 1.5 2.0 2.5 3.0

CAPCAP/C

2Np

CAP/C6Np

F 884 (C

AP)

Draw ratio

T10 MPa

T100 MPa

(A)

0

0.1

0.2

0.3

0.4

1.0 1.5 2.0 2.5 3.0

CAP/C2Np

CAP/C6Np

F 772 (a

dditi

ve)

Draw ratio

T10 MPa

T100 MPa

(B)


0

1

2

3

1.0 1.5 2.0 2.5 3.0 3.5

T10MPa

T100MPa

F 772 /

F 884

Draw ratio

CAP/C2Np

0

1

2

3

1.0 1.5 2.0 2.5 3.0 3.5

T10 MPa

T100 MPa

F 772 /

F 884

Draw ratio

CAP/C6Np


0

1

2

1.0 1.5 2.0 2.5 3.0 3.5

T100 MPa

T10 MPa

F 772 /

F 2860

Draw ratio

CAP/C6Np

Nobukawa et al. S1

Supplementary information

The effect of flexible chains on the orientation dynamics of small molecules dispersed in

polymer films during stretching

Shogo Nobukawa*, Yoshihiko Aoki, Yoshiharu Fukui, Ayumi Kiyama, Hiroshi Yoshimura,

Yutaka Tachikawa, and Masayuki Yamaguchi

*Corresponding author

Affiliation: Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,

Ishikawa 923-1292, Japan,

E-mail: [email protected]

Orientation function determined by polarized FT-IR spectroscopy

In order to evaluate the degree of orientation for CAP, C2Np and C6Np in the

stretched films, polarized FT-IR spectra were measured. Intensities of absorption peaks for

CAP, C2Np and C6Np were evaluated. The assignments of the peaks were summarized in

Table S1.

The peak at 772 cm-1 is assigned to the out-of-plane bending of the aromatic ring

C-H groups for C2Np and C6Np 1, while the peak at 884 cm-1 are assigned to asymmetric

stretching mode of a pyranose ring or wagging mode of ring C-H bonds.2,3 The peak at

806 cm-1 is regarded as the parallel vibration mode to the chain axis from the FT-IR data

although the assignment has not been reported. Asymmetric (asym.) and symmetric (sym.)

stretching vibrations for C-H in methylene group4 are observed at 2800-3000 cm-1 for CAP,

C2Np and C6Np. The sym. stretching peak of C-H bond for C6Np at 2860 cm-1 can be

decomposed from that for matrix CAP at 2884 cm-1, although the asym. peaks for both

components cannot be separated due to the overlapping.

The polarized FT-IR spectroscopy gives the information of molecular orientation in

stretched polymer films.5 Two spectra parallel and perpendicular to the stretching direction

are obtained from the FT-IR measurement. Since the intensity of absorption peak is

Nobukawa et al. S2

dependent on the orientation direction of each transition moment, the dichroic ratio,

R = A// / A⊥, is related to the orientation function, F, as follows,

21

21

0

0

+−

−+

=RR

RRF (S1)

Here, A// and A⊥ are peak intensities of polarized spectra parallel and perpendicular to the

stretching direction, respectively. R0 = 2cot2β, where β is the angle between the transition

moment corresponding to the absorption band and the chain axis. The angle β for each band

can be determined from the FT-IR spectra simulated along parallel and perpendicular

directions to the chain axis by using density functional theory (DFT/B3LYP) with

6-31+G(d,p) basis set. As the result, the angles β for CAP were estimated to be 68.6 and 0 o

at 884 and 806 cm-1, respectively. On the other hand, β for C2Np and C6Np was determined

to be 90 o at 772 cm-1. The angle β for the asymmetric C-H stretching vibration is also 90 o

at 2840 cm-1 only for C6Np.

Analysis of the absorbance for each peak

Figures S1 and S2 show polarized FT-IR spectra for CAP and CAP/additive films as

examples. As seen in Figure S1, the absorbance, A, for C-H bending vibrations of CAP,

C2Np and C6Np is determined by drawing baseline. In contrast, A for C-H stretching mode

of C6Np is estimated as the peak area by deconvolution of the spectrum with the sum of five

Lorenz type functions represented by,

( ) ∑ +−=

5

22)(i ii

ii

BBAABS

ννπν (S2)

Here, Ai, Bi, and ν i are surface area, half width, and absorption wavenumber for ith peak.

Examples are shown in Figure S2. For the deconvolution of the spectra, Ai was a variable

while Bi, and ν i were used as constant values. As seen in the figure, the fifth peak at

2860 cm-1 is observed only in CAP/C6Np film. By applying the obtained absorbance A for

each peak to equation S1, the orientation function for CAP, C2Np and C6Np can be estimated.

Nobukawa et al. S3

Table S1. Peak assignments in FT-IR spectra for CAP, C2Np and C6Np.

out-of-plane bending

aromatic C-H stretching or wag. of C-H (pyranose)

C-H stretching frequency 4, ν / cm-1

-CH3, asym. -CH2-, asym. / sym.

CAP* - 884, 806 2,3 2981 2943 / 2884

C2Np** 772 1 - - 2956 / 2884 C6Np** 772 1 - - 2940 / 2860

* in bulk

** in chloroform solution

0

0.2

0.4

0.6

0.8

1.0

1.2

7007508008509009501000

perpendicularparallel

ABS

Wavenumber /cm-1

DR = 1.0CAP/C

2Np

(A)

baseline

0

0.2

0.4

0.6

0.8

1.0

1.2

7007508008509009501000

perpendicularparallel

ABS

Wavenumber /cm-1

DR = 2.5CAP/C

2Np

(B)

baseline

Figure S1. Polarized FT-IR spectra of CAP/C2Np film (A) unstretched and (B) stretched with a draw ratio of 2.5. Baselines for evaluation of orientation functions are drawn in the figures.

Nobukawa et al. S4

0

0.5

1.0

1.5

27002800290030003100

12345total

AB

S

Wavenumber /cm-1

DR = 1.0

CAP (0o)

(A)

0

0.5

1.0

1.5

2.0

27002800290030003100

12345total

ABS

Wavenumber /cm-1

DR = 1.0

CAP/C6Np (0o)

(B)

1: 2982 cm-1 asymmetric stretching mode (CH3 in CAP)

2: 2944 cm-1 asymmetric stretching mode (CH2 in CAP, C2Np, C6Np)

3: 2920 cm-1 asymmetric stretching mode (CH2 in CAP, C2Np, C6Np)

4: 2883 cm-1 symmetric stretching mode (CH2 in CAP, C2Np, C6Np)

5: 2860 cm-1 symmetric stretching mode (CH2 in C6Np)

Figure S2. Deconvolution of polarized FT-IR spectra of (A) CAP and (B) CAP/C6Np films with a polarized angle = 0o. The order of peak number is from high to low wavenumber.

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the effect of flexible chains on the orientation

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